A composite may consist of matrix plastics and reinforcing fibers. The excellent strength values of structural composites are based on the high-module reinforcing filaments, such as glass, aramid and carbon fibers. At the present, primarily thermosetting plastics with good dimensional stability and chemical and thermal resistance are used as matrices of such composites. Epoxies dominate in structural, primary applications, for example, the aeronautic and space industries, whereas secondary applications for example in the automobile industry, involve also polyesters, phenols and vinyl esters. Themoplastic matrices will become more commonly used with the development of new technical polymers, such as polyphenylene sulphide (PPS), polyether-ether ketone (PEEK) and polyther sulphone (PES), primarily furthered by the excellent physical and chemical properties of these plastics.
For the part of the reinforcement structures, the composites can be based either on continuous or staple fibers. The continuous fibers form a uniform, usually uniaxial network. The staple fibers are divided by length into short ones, i.e., .ltoreq.1 . . . 3 mm, and long ones, i.e. about .ltoreq.3 . . . 10 mm. In structural composites of continuous fibers, bearing a static and dynamic load, four basic factors are to be taken into account:
the fibers, PA1 the matrix, PA1 the ordering degree and orientation of the fibers, and PA1 the bonds between the fibers and the matrix. PA1 uniaxially, PA1 biaxially, i.e. in a plane, PA1 triaxially, i.e. in three dimensions.
The reinforcing fibers receiving the load determine the strength and rigidity of the structural composite. They also toughen the material by absorbing brittling energy, for example by a gliding mechanism between the binding surfaces. The matrix protects and supports the fibers, particularly in a pressing situation, as well as transmits the force on the piece from one fiber to another. In a situation of overloading, the matrix must be capable of transmitting the force between the broken filaments by means of shear between the material layers, so that the fibers could bear a traction load again. For this purpose, the fiber length must exceed a critical value. The quality of the composite is determined by how evenly the fibers are distributed in the structure and how well they are moistened by the matrix. The latter feature is essentially dependent on the fluidity, that is, the viscosity, of the matrix. The fibers can be oriented in three ways:
In the case of structural composites, uniaxial continuous-fiber laminates represent the first group, woven fabrics the second group, and, for example braids the third group. In the end, it is the interface between the matrix and the fibers that determines how successful the composite is. It is the aim to create a strong bond between the fibers and the matrix in order to eliminate pores in the interface. In some cases, the fibers must be coated with a binding agent in order to secure the connection of the reinforcement to the matrix. Thus, there are, in fact, two interfaces in the structure, i.e. between the binding agent and the fiber and between the binding agent and the matrix.
The excellent mechanical properties of the structural composites are due to the continuous fibers, or filaments, which are usually 50 times stronger and 20 to 150 times more rigid than the matrix materials. Fibers with low density (1.44-2.7 g/cm.sup.3) have high tensile strength and elastic modulus (3.0-4.5 GPa and 80-550 GPa, respectively), whereas the corresponding typical values for matrix polymers are 30-130 MPa and 2.0-4.0 GPa, respectively. During formation of the fibers, the strength of the material increases with the rise of the axial orientation of the crystals and with the decrease of defects (such as cracks and dislocations) in the microstructure. One-dimensional continuous-fiber composites are therefore much stronger in the longitudinal than in the transverse direction. It is an anisotropic material whose properties depend on the direction. The composite can thus be dimensioned according to the prevalent loading situation with a minimum material waste.
The properties of the composite are anisotropic, which is clearest in uniaxial structures. They give maximum tensile strength and modulus. As pieces are usually loaded by a three-dimensional stress field, the one-dimensional plates must be laminated on top of each other in order to achieve reinforcement in several directions. When the continuous fibers are oriented multiaxially in a plane, pseudoisotropic laminates are obtained. In three-dimensional basic coordinates, their stiffness in the xy-plane is comparable to that of aluminum mixtures, but the transverse tensile strength and elastic modulus as well as the shearing strength are low. This is due to the differences in the elastic coefficients between different layers, and, therefore, the load of the matrix varies in the direction of the thickness of the laminate (z). Thus the breaking of the structural composite is in most cases due to the gliding of layers in relation to each other.
For orientation of the continuous fibers in the structure in a desired way, thermosetting plastic matrices with low viscosity, such as epoxies and polyesters, and expensive manual methods, such as manual lamination and autoclaves, have generally been used in the production of complex composite pieces. The manufacture of thermosetting plastic composites has been slow and difficult, because special equipment is required for the storage and handling of a fluid matrix and the chemical cross-linking of the composites takes a long time. Other difficulties, and in some cases even barriers, for the future development of thermosetting plastic composites are caused by their brittleness and sensitivity to moisture as well as questions of occupational safety.
The use of thermoplastics as matrices for composites has been delayed by the view that it is impractical to moisten densely packed continuous fibers by viscose polymers. Also, there have been doubts on the succeeding of thermoforming in structures containing more than 60 vol-% inextensible fibers. However, the excellent physical properties of the new technical thermoplastics are gradually changing these attitudes.
The processing of thermoplastic composites is based on heat and pressure, so that they are considerably faster to manufacture than corresponding thermosetting plastic composites. Because thermoforming and/or pressure forming can be repeated several times, broken structures can be easily repaired. In addition, scrap structures and finishing waste can be used, for example e.g. as material for injection molding.
Thermoplastic staple fiber composites are manufactured by conventional melt working methods of polymers, for example by injection moulding, so that the orientation of the reinforcements cannot be fully controlled. In injection molding, for example, the orientation of reinforcing staple fibers and polymer molecules is influenced at the filling stage of the mold by a complex flow field with both a shearing and an extending component. When the short staple fibers are fluid-impregnated with a thermoplastic in the screw of the extruder, they are broken down in the strong shearing and extending flow because of a mutual attrition. The viscosity of technical thermoplastics being 10.sup.3 to 10.sup.6 times higher than that of thermosetting plastics, the polymer cannot fully moisten the whole surface area of the fibers. The reinforcements are thus rubbed further at the injection molding stage, resulting in an average fiber length of 180 to 200 .mu.m in a finished product.
The published European Patent Application No. 272 083 discloses a method for manufacturing a porous piece for use as a reinforcing material by piling up fabrics formed of yarns comprising matrix and reinforcing fibers and by binding them together with the application of heat. The structure is used for thermosetting plastics applications. Further, from the published European Patent Application No. 133 893, a method is known for manufacturing shaped pieces reinforced with fibers in a similar way from woven or knitted fabrics piled on top of each other.
In the pieces presented above, however, all the structural possibilities have not been taken into account in the z-direction of the piece.